Investigators in Russia are looking into what caused two passenger planes to crash at almost the same time while flying from Moscow to cities in the southern part of the country. Eighty-nine people died in the two overnight air disasters.

Search and rescue teams are sifting through the wreckage where the two planes crashed after disappearing from radar screens at almost the same time.

The smaller of the two airliners, a Tupolev-134, crashed in the Tula region about 180 kilometers south of Moscow while on a flight to the city of Volgograd.

The other plane, a Tupolev-154, went down in the Rostov-on-Don region further south near the Black Sea.

The near-simultaneous nature of the crashes immediately raised suspicion that the two planes were either hijacked or brought down by onboard bombs.

There was an unconfirmed report that the larger plane sent out a distress signal just before it went down.

Upon hearing of the disasters, President Vladimir Putin put the main internal security service, or FSB, in charge of the investigation.

FSB officials say so far they haven't found any concrete evidence that the crashes were due to terrorist acts. They say mechanical failure or violation of safety rules are other possibilities.

Investigator Gennady Skachkov says the wreckage at one of the crash sites appears to rule out the possibility of a major explosion.

"The aircraft is almost whole and the trail of wreckage is not spread over a wide area," he said. "The plane didn't send out any distress signal, and we really don't understand how the crash could have happened."

The planes took off within an hour of each other from Moscow's Domodedyevo airport, one of five commercial air facilities serving the Russian capital. The airport is one of the most modern in Russia, and security measures have long been in place there.

However security concerns in Russia are high due to the ongoing war in the breakaway region of Chechnya, where Russian troops have been battling separatist fighters for most of the past decade.

There have been bombing incidents in Moscow and other cities in recent years attributed to Chechens.

This coming Sunday an election is to be held in Chechnya to replace the former president who was assassinated by separatists in a bomb blast in May.

Chechnya's most prominent political leader, Aslan Maskhadov, has denied any involvement in the air crashes.